Micro-fluid ejection heads found in micro-fluid ejection devices, such as ink jet printers continue to technologically evolve. These micro-fluid ejection heads are made with nozzle members, such as nozzle plates, containing flow features. One technological advance in making improved micro-fluid ejection heads is the use of a thick photoresist layer applied to a device surface of the substrate. The thick film layer is imaged to provide the flow features for the micro-fluid ejection heads. Use of the photoimageable thick film layer enables more accurate alignment between the flow features and ejection actuators on the device surface of the substrate. However it is also important that the photoresist layer is capable providing good imaging performance including crisp images, high aspect ratio, straight side walls, and a large processing window. Moreover the thickness of the thick film needs to be between 5 microns and 30 microns.
However there are environmental problems with the photoresist layers found in the micro fluid ejection heads in the market today, namely the existence of the harmful element antimony. Antimony and its compounds are considered to be hazardous and harmful pollutants by the United States Environmental Protection Agency and the European Union. Acute or short term exposure to antimony by inhalation in humans results in effects to the skin and eyes. Chronic exposure to antimony can lead to inflammation of the lungs, chronic bronchitis, and chronic emphysema. Waterborne antimony pollutant is very toxic. Ingesting contaminated water can cause severe sickness. In fact the European Union is even considering a future ban of products containing antimony because of the potential health risks associated with antimony. Accordingly, there is a need for an antimony-free photoresist formulation that is environmentally friendly.
Another detriment to using photoresist layers containing an antimony based photoacid generator is the existence of hydrofluoric acid in the photoresist layer. Many photoresist layers are manufactured with a hexafluoroantimonate photoacid generator. The decomposition of SbF6− ion generates hydrofluoric acid which remains in a micro-fluid ejection head such as an ink jet printhead. Since hydrofluoric acid is very corrosive, erosion of the wiring of the printhead could occur. Hydrofluoric acid contaminated ink channels can induce ink settlement and more seriously block the ink channels on the flow features on the printhead. The existence of hydrofluoric acid can thereby lead to a failure of the printhead.
As explained above, flow features are photo lithographically processed. The photoresist layer containing an antimony based photoacid generator has to be ‘bumped’ with an overdose of ultraviolet light and ‘baked’ in a convection oven at 200 C for 2 hours. The ‘bump and bake’ step is necessary to drive out the undesirable hydrofluoric acid. Unfortunately, the high thermal stress induced on the photoresist layer during the bump and bake process has proven to be detrimental from a chemical, physical, and mechanical properties standpoint. It would be beneficial from a manufacturing standpoint to have a printhead that would not need to go through this harsh bump and bake process. Accordingly, it would be beneficial to have a photoresist formulation that does not contain hydrofluoric acid.
There is provided a thick film layer as ink flow feature for a micro-fluid ejection head. The thick film layer includes a negative photoresist layer derived from a composition containing a multi-functional epoxy compound, a first and second difunctional epoxy compound, a methide-based photoacid generator that does not contain antimony, a chromophore, and an aryl ketone solvent. Optionally the photoresist layer contains an adhesion enhancer. The methide-based photoacid generator is i-line sensitive. Use of an i-line sensitive photoacid generator also alleviates the need to use a photosensitizer in the thick film layer. The absence of the photosensitizer keeps down the cost of manufacture of the thick film photoresist layer. The negative photoresist layer is environmentally friendly and provides improved image performance including flow features with crisp images, straight or vertical side walls and a large processing window. The environmentally friendly thick film photoresist layer is derived from a low molecular weight polymeric difunctional epoxy component having a weight average molecular weight ranging from about 2500 to about 4000 Daltons, a monomeric difunctional epoxy component, a multifunctional epoxy resin, a tris-aryl-sulfonium tris(trisfluoromethanesulfonyl) methide, i-line sensitive photoacid generator, an aryl ketone solvent, a chromophore, and an optional adhesion enhancing component. Another formulation which may be used to provide an environmentally friendly thick film layers is derived from a low molecular weight polymeric difunctional epoxy component having a weight average molecular weight ranging from about 2500 to about 4000 Daltons, a monomeric difunctional epoxy component, a multifunctional epoxy resin, a tris-aryl-sulfonium tris(trisfluoromethanesulfonyl) methide, i-line sensitive photoacid generator, an aryl ketone solvent. For purposes of this disclosure, “difunctional epoxy” means epoxy compounds and materials having only two epoxy functional groups in the molecule. “Multifunctional epoxy” means epoxy compounds and materials having more than two epoxy functional groups in the molecule.